In May 1952, Rosalind Franklin and her graduate student Raymond Gosling prepared a thin fibre of hydrated calf-thymus DNA at King’s College London and exposed it to X-rays for 62 hours. King’s records that the camera was set up on 2 May and the photograph was developed on 6 May. The resulting image, numbered Photograph 51 in their experimental series, showed the wet or B form of DNA with exceptional clarity.
The DNA itself had an important history. Swiss chemist Rudolf Signer had produced unusually pure DNA from calf thymus and distributed samples at a scientific meeting in London. A historical account published in CHIMIA says Maurice Wilkins received one of those samples and that its quality enabled Franklin’s decisive X-ray work.
What Photo 51 actually shows
Photo 51 is not a conventional photograph of a DNA molecule. X-rays scattered by the atoms in many similarly aligned DNA fibres interfered with one another, producing dark spots on sensitive film. Repeating molecular structures create regular diffraction patterns, and a helix viewed from the side produces the characteristic cross visible in the image.
The spacing of the spots carried additional information. The vertical separation within the cross indicated ten stacked bases per helical turn, while the broad reflections above and below the centre reflected the regular stacking of those bases. The absent fourth spot in each arm helped Franklin infer that two chains would be displaced from one another by three-eighths of the helix’s pitch.
Franklin understood that the wet B form was helical, but she concentrated much of her effort on the drier A form. Its diffraction pattern was more complicated, yet its greater crystalline order promised richer structural information to a meticulous crystallographer. That choice slowed her route to a complete model, but it was not evidence that she failed to understand Photo 51.
How the photograph reached Watson
By early 1953, Franklin was preparing to leave King’s for Birkbeck College. Gosling was returning to Wilkins’s supervision and gave Wilkins a print of Photo 51 during the handover. When James Watson visited King’s in January, Wilkins showed him the image.
Franklin did not know that this specific photograph had been shown to Watson. Wilkins, according to King’s account, believed Watson had already seen earlier diffraction patterns showing a helical cross. Watson immediately recognised Photo 51 as powerful evidence for a helix, although the photograph alone did not supply the chemical logic of the finished double-helix model.
The original article’s citation to Science News was attached to the claim that Watson instantly understood the structure and carried its dimensions back to Cambridge. That is not what the source concludes. Science News instead reports a newer archival interpretation in which Franklin knowingly participated in a wider exchange of data, even though she was unaware that Wilkins had shown Watson Photo 51.
The second route by which the data travelled
Photo 51 was only one source of information. Max Perutz, head of the Medical Research Council unit at Cambridge’s Cavendish Laboratory, had received an internal report on the King’s biophysics group during an official MRC visit. The report included measurements from Franklin’s work, and Perutz later made it available to Watson and Francis Crick.
Those measurements helped Watson and Crick test possible models. They did not amount to a completed double-helix solution, and the surviving evidence does not support presenting the antiparallel arrangement as a finished conclusion that Franklin had simply written down for them.
The ethics of this exchange are still debated, but the archival record does not fit a clean story of secret theft. In a 2023 Nature commentary, historians Matthew Cobb and Nathaniel Comfort used an overlooked letter and an unpublished 1953 article to argue that Franklin was an equal participant in a loosely collaborative effort. That interpretation does not erase the fact that she was not told about Wilkins showing Watson the photograph.
What Franklin had worked out independently
Franklin’s notebooks and early-1953 manuscript show that she was closing in on the B-form structure through her own analysis. She had established that the phosphate groups lay on the outside of the molecule, contrary to Linus Pauling’s proposed triple-helix model. She had the approximate diameter, the pitch and the number of bases per turn.
By March, she regarded a helical structure as highly probable and was considering a double helix with ten bases per turn. Her approach remained measurement-first: she wanted the diffraction data to determine the structure before committing to a physical model.
Watson and Crick worked differently. They combined experimental dimensions with chemical constraints, Erwin Chargaff’s base ratios and advice from crystallographer Jerry Donohue. Their strength was rapid synthesis, but that synthesis depended on evidence produced by several laboratories, including Franklin and Gosling’s work at King’s.
The three Nature papers
On 25 April 1953, Nature published three consecutive papers. Watson and Crick’s model appeared first, followed by a paper from Wilkins, Alec Stokes and Herbert Wilson, then Franklin and Gosling’s diffraction analysis containing Photo 51.
That sequence made the two King’s papers look like confirmation of a model already completed in Cambridge. In their own paper, however, Watson and Crick conceded that their structure was not yet rigorously proved and required comparison with more exact experimental results.
Their acknowledgement was broad rather than specific. In the original Watson and Crick paper, they said they had been stimulated by the general nature of the unpublished results and ideas of Wilkins, Franklin and their colleagues. That wording did not tell readers exactly which measurements had influenced the model.
Her work after DNA, and what caused her death
Franklin moved to Birkbeck College in 1953 and built a productive research programme on viruses. Her team made important advances on tobacco mosaic virus and began work on poliovirus. The US Environmental Protection Agency’s biographical account records that her poliovirus crystallography ended when her cancer progressed and that colleagues continued the work after her death.
Franklin was diagnosed with ovarian cancer in 1956 and died on 16 April 1958 at the age of 37. It is tempting to connect her illness to years of X-ray research, but the surviving historical record cannot establish the cause of an individual cancer. The claim that her equipment “probably killed her” goes beyond the available evidence.
In 1962, the Nobel Prize in Physiology or Medicine went to Crick, Watson and Wilkins for their work on nucleic-acid structure. Franklin had been dead for four years and could not be considered under the Nobel nomination practice. Her absence from the award should not be confused with an absence from the discovery itself.
Why the drawer still matters
The drawer remains a powerful image because it captures the difference between two scientific styles. Franklin set the B-form photograph aside while pursuing the more difficult A form through careful mathematical analysis. Watson and Crick built provisional models quickly and tested them against every piece of evidence they could obtain.
Yet the drawer should not become a device for turning Franklin into someone who possessed the answer without understanding it. She understood the helical implications of the B form and independently developed many of the structure’s essential dimensions. The unresolved question is not whether she recognised the evidence, but how credit should be assigned when experimental work and model building crossed institutional boundaries.
What the cross on the film meant
The cross at the centre of Photo 51 encoded the geometry of B-form DNA. It pointed to a helix roughly two nanometres wide, with stacked bases separated by about 0.34 nanometres and a full turn spanning about 3.4 nanometres. The 1953 model described ten base pairs per turn; modern measurements usually place B-DNA closer to 10.5 under typical conditions.
Those dimensions helped reveal a molecule with sugar-phosphate backbones on the outside and paired bases inside. The two strands run in opposite directions, and their complementary base sequence provides the molecular basis for copying genetic information in cellular life.
The afterlife of Photo 51
Franklin’s name now belongs to Rosalind Franklin University in North Chicago, the Rosalind Franklin Institute in Britain and a European Mars rover. Reproductions of Photo 51 appear at King’s and in museums and textbooks worldwide, but the original article’s claim that a postcard-sized glass plate is displayed at King’s is not supported.
The surviving material includes photographic prints rather than a publicly displayed original glass plate. The Science History Institute holds an original print annotated by Franklin and Aaron Klug in its History of Molecular Biology Collection in Philadelphia.
The space connection is now approaching its next chapter. ESA schedules the Rosalind Franklin mission for launch in late 2028, with arrival at Mars roughly two years later. The rover is expected to land at Oxia Planum and drill as deep as two metres for samples protected from surface radiation, searching for organic molecules and other possible signs of past or present life.
Photo 51 does not tell a simple story of a scientist who missed what was sitting in front of her. It marks the moment evidence produced by Franklin and Gosling entered a model being built elsewhere before Franklin knew that the specific image had crossed the corridor, then the distance between London and Cambridge.
The drawer matters because someone else opened it. The fuller history matters because Franklin already knew there was something important inside.
